1. Field of the Invention
This invention relates to an improved process for preparing defluorinated phosphate rock granules that can be employed as animal feed from fluorine-containing phosphate rock.
2. Description of the Prior Art
Phosphate-containing materials are in great demand for use as animal feed supplements, and the market for such materials is constantly growing. The principal sources of phosphate in the United States are the great natural deposits of pebble rock and phosphate rock found in Florida and in the western states, and such widely distributed phosphatic materials as apatite. However, these naturally occuring phosphate materials contain combined fluorine in quantities which can, under certain conditions, be detrimental to health when used as animal feed supplements. As a result, when these phosphatic materials are to be used as animal feed supplements, the phosphatic material is commonly treated to reduce the fluorine content of the material usually below a specified minimum amount. For instance, the fluorine content of an acceptable animal feed supplement today should be less than one part of fluorine per 70 parts of phosphorus by weight. In addition, industry specifications require that the phosphorus content of the defluorinated phosphate rock be at least 18% by weight.
A great deal of effort has been devoted to the problems of developing economical and efficient processes for reducing the fluorine content to acceptable limits while maintaining an acceptable level of phosphorus content in these phosphatic materials (herein collectively referred to as phosphate rock). Thermal processes have been the most widely known and commercially successful means of defluorinating phosphate rock. A typical thermal process involves calcining the phosphate rock in the presence of phosphoric acid, sodium carbonate, water vapor and other reagents at a temperature in excess of about 1000.degree. C. to drive off a substantial portion of the fluorine content of the rock and thereby obtain a defluorinated phosphate product. The principal difficulty encountered in the defluorination of phosphate rock by high temperature calcination arises from the fact that the temperature required for the removal of fluorine is so high that objectionable fusion or sintering of the rock can take place before removal of fluorine to the desired extent is effected. In order to avoid the disruption of the calcination process due to partial fusion and the consequent formation of large lumps of phosphate-containing material in the kiln or other calcining apparatus, it has heretofore been the practice to form the defluorination feed material into nodules, agglomerates, or granules having sufficient mechanical strength, porosity, and resistance to fusion to withstand calcination in a rotary kiln, and a great deal of time and effort have been expended in developing methods for making such a granulated feed material. The prior art practice of making defluorination feed material is exemplified by the processes described in U.S. Pat. No. 2,995,436 and U.S. Pat. No. 3,189,433 both issued to Hollingsworth et al, and co-pending U.S. patent application Ser. No. 761,049, filed Jan. 21, 1977 to Harold V. Larson.
However, the problem of containing defluorinated phosphate rock products that have the acceptable level of at least 18% by weight phosphorus content still remains. The supply of relatively pure phosphate rock concentrate, like many other raw materials today, is rapidly diminishing. Furthermore, costs have increased for phosphate rock concentrates which contain, for example, greater than about 72% BPL (Bone Phosphate of Lime) by weight and less than 4% by weight acid insolubles. In an effort to maintain acceptable profit levels, animal feed producers have resorted to using lower cost, less pure phosphate rock concentrate which contain, for example, from about 68% to 70% BPL by weight and from about 6% to about 10% acid insolubles by weight. Although such lower grade phosphate rock materials can be successfully defluorinated by conventional defluorination techniques, the excess acid insoluble content of the starting phosphate rock adversely dilutes the final product and makes it difficult to produce a defluorinated phosphate rock having a phosphorus content in excess of about 18% by weight.
Another problem facing animal feed producers is that in the heating or defluorination step, some of the P.sub.2 O.sub.5 present in the raw material may unexpectedly volatilize off with the fluorine, and, therefore, the resulting phosphorus content may be lower than the acceptable 18% level.
To overcome these problems of phosphorus content deficiency in animal feed products, the industry has added increasing amounts of phosphoric acid to the phosphate rock concentrates. Normally, the phosphoric acid is added to the rock concentrate in a mixing step prior to the granulating and heating or defluorination steps. However, it has been realized that only so much phosphoric acid can be added prior to the heating step. If the relative amount of added phosphoric acid is too great, certain processing problems such as over-size defluorinated phosphate rock granules may result. Therefore, producers are limited by the amount of phosphoric acid they can add prior to the heating or defluorination step, and, in some instances, such resulting products are unfortunately below the 18% by weight phosphorus content requirement.
U.S. Pat. No. 3,264,086, issued to Hollingsworth et al. on Aug. 2, 1966 teaches treating certain phosphatic compounds with phosphoric acid to raise the phosphorus content therein up to 27% by weight. This treatment generally can be carried out by spraying phosphoric acid on dry phosphate raw material at a temperature not above 260.degree. F. (127.degree. C.). While this patent appears to teach an acceptable way of increasing phosphorus content in animal feed products, it has the drawback of being an additional processing step, and, therefore, increasing the cost of product.
The present invention is therefore directed toward an improved method for producing defluorinated rock which has an acceptable phosphorus content of at least 18% whereby additional phosphoric acid is added to defluorinated phosphate rock granules after the defluorination step. However, the present invention has the advantage of adding the phosphoric acid during an existing processing step, and, therefore, this addition will not raise the product cost excessively.